The tractive effort figures given for a locomotive (steam or diesel) are only the theoretical pull developed at very low speeds, and by itself is pretty much a worthless figure. Most steam locomotives and early diesels list a tractive effort based on 25% of the weight on drivers.
For steam locomotives, the cylinder horsepower is generally designed to be at this figure. Once moving, at some point, depending on the design, the cylinders demand more steam than the boiler can produce. This requires the johnson bar to be "hooked up" or moved towards center limiting the time steam is admitted to the cylinders, resulting in a substantial drop in tractive effort/drawbar pull.
The 25% figure for a diesel-electric (later 30%, now 40%+ with modern AC drive or electronic wheel slip systems) is the starting tractive effort, an indication of the maximum TE it can theoretically develop before the drivers lose adhesion and slip. As speed increases, the tractive effort drops off dramatically. In addition, there is a minimum continuous speed that it must be operated at to staay within the limits of its electrical system (to prevent overheating). In addition, a diesel engine's nominal horsepower rating is only up to 1500 feet elevation and must be derated at higher altitudes.
The TE curve for a diesel-electric locomotive can be calculated by knowing the traction horsepower (generator input), and the continuous rating is often pblished for each model. Referring to a DE loco as a "25 ton", "44 ton", etc. suggests nothing but the starting tractive effort and low speed short time capability.
For a steam locomotive, to calculate the true TE for a given speed you have to know the boiler capabilities. Generally, the bigger the firebox/heating surface ratio to the TE the more a loco will pull at speed. THis is one reason the trailing truck design became popular, a 2-8-2 has greater boiler horsepower than a 2-8-0 with the same TE, etc.
Regardless, you have to subtract the tractive effort required to move the locomotive (and tender) itself. While a high weight on drivers is a good thing while at low speeds, at a higher speed it becomes dead weight that must be drug up the hill.
Theoretical approximate tractive efforts for the Loop diesels:
#? GE 25T 12,500# starting 3,100# @10 mph
#21 GE 44T 22,000# starting 9,200# @10 mph
#1203 Porter 40,000# starting 13,500# @10 mph
The above figures are only a guess since I do not know the true horsepower input. Starting tractive effort can vary from 20% to 30+% depending on adhesion conditions. Note that the TE at 10 mph is far less than the maximum tractive effort often listed.
